Neisseria meningitidis is a Gram-negative bacterium and the causative agent of meningococcal meningitis and septicemia. Its only known host is the human, and it may be carried asymptomatically by approximately 10% of the population (Caugant et al, 1994, J. Clin. Microbiol. 32 323).
N. meningitidis may express a polysaccharide capsule, and this allows classification of the bacteria according to the nature of the capsule expressed. There are at least twelve serogroups of N. meningitidis: A, B, C, 29-E, H, I, K, L, W135, X, Y and Z, of which serogroups A, B, and C cause 90% of meningococcal disease (Poolman et al, 1995, Infect. Agents and Dis. 4 13). Vaccines directed against serogroups A and C are available, but the serogroup B capsular polysaccharide is poorly immunogenic and does not induce protection in humans.
Other membrane and extracellular components are therefore being examined for their suitability for inclusion in vaccines. Examples include the outer membrane proteins of classes 1, 2 and 3 (porin; encoded by por genes) and classes 4 (Rmp) and 5 (Opacity proteins; encoded by opa and opc genes). However, N. meningitidis is very effective at evading immune responses by antigenic and phase variation. For example, the Opc protein is an adhesin/invasin (Virji et al., 1995, Mol Microbiol. 18 741-54) that is highly immunogenic (Wiertz et al., 1996, Infect Immun. 64 298-304), yet its expression is phase-variable (Sarkari et al., 1994, Mol Microbiol. 13 207-17), and by diversion—generation of immune responses against hyperimmunogenic moving targets, in particular PorA.
PorA is highly variable between strains and generates an immune response in both patients and asymptomatic carriers, to the extent that it has been used as a marker for strain identification, representing the serosubtype system (McGuinness et al., 1990, J Exp Med. 171 1871-82). PorA is a key antigen, and has been used in previous effective and registered vaccine formulations and is considered an ideal antigen to elicit effective bactericidal antibodies. However, strain-to-strain variability in surface loops results in a variable target, and vaccines are typically PorA type-specific. Although efforts have been made to generate multivalent PorA vaccines covering up to six different PorA types (van der Voort et al., 1996, Infect Immun. 64 2745-51), this target has been judged to be too variable, and recent vaccine development has moved away from this antigen primarily for that reason.
The current model of PorA monomer topology indicates eight extracellular loops (Derrick et al., 1999, Infect. Immun. 67 2406-13; van der Ley et al., 1991, Infect. Immun. 59 2963). The longest loops (1 and 4) are the most variable, hence are referred to as Variable Region 1 (VR1) and Variable Region 2 (VR2). Less variability is seen in loops 5 and 6 (semi-variable SVR1 and 2 respectively), with essentially no variability in the remaining loops. Loop 3 is predicted to form a “plug” in the pore formed by each subunit of the PorA trimer. Even within VR1 and VR2, most of the variability is confined to residues predicted to form the tip of each loop. Indeed, in both mice and in immunized human volunteers, epitope mapping showed that the majority of the antibody response is directed at the “top” of loops 1 and 4, the region that is variable between strains (van der Voort, et al., 1997, FEMS Immunol. Med. Microbiol. 17 139-48), presumably explaining the strain specificity of anti-PorA responses.